The multifunctional cytokine transforming growth factor-β (TGF-β) plays major roles in the biology of immune, endothelial, epithelial, and mesenchymal cells during development and adult life in invertebrate and vertebrate species. In mammals, these functions are mediated by three isoforms, TGF-β1, 2, and 3, which are each widely expressed. All three isoforms interact with the same cell surface receptors (TGFBR2 and ALK5) and signal through the same intracellular signaling pathways, which involve either canonical (i.e., SMADs) or noncanonical (i.e., MAPK, JUN, PI3K, PP2A, Rho, PAR6) signaling effectors. The canonical TGF-β signaling pathway, whereby TGF-β signaling is propagated from the TGF-β receptor apparatus through phosphorylation of cytoplasmic SMAD-2/3, complex formation with SMAD-4, nuclear translocation of the SMAD-2/3/4 complex, and binding to SMAD response elements located in the promoter regions of many genes involved in the fibrogenic response, has been the most intensively studied. However, despite having similar signaling partners, each isoform serves individual biological functions, perhaps due to differences in binding affinity to TGF-β receptors, activation mechanism, signaling intensity or duration, or spatial and/or temporal distribution.
Knockout and conditional deletion models of TGF-β isoforms, receptors, and signaling mediators, as well as function-blocking reagents targeting all TGF-β isoforms, have revealed essential roles for TGF-β in T-cell, cardiac, lung, vascular, and palate development. For instance, mice deficient in TGF-β1 either die in utero owing to defects in yolk sac vasculogenesis or are born and survive into adult life but develop severe multiorgan autoimmunity. Genetic deletion of TGF-β signaling mediators has shown an essential role for Smad2 in early patterning and mesodermal formation, and mice lacking Smad3 are viable and fertile, but exhibit limb malformations, immune dysregulation, colitis, colon carcinomas, and alveolar enlargement. In adult tissues, the TGF-β pathway is thought to regulate the dynamic interactions among immune, mesenchymal, and epithelial cells to maintain homeostasis in response to environmental stress.
The normal homeostatic pathways mediated by TGF-β are perturbed in response to chronic repetitive injury. In cases of injury, TGF-β becomes a major profibrogenic cytokine, delaying epithelial wound healing by inhibiting epithelial proliferation and migration and promoting apoptosis and expanding the mesenchymal compartment by inducing fibroblast recruitment, fibroblast contractility, and extracellular matrix deposition. Indeed, intratracheal transfer of adenoviral recombinant TGF-β1 to the rodent lung dramatically increases fibroblast accumulation and expression of type I and type III collagen around airways and in the pulmonary interstitium, and neutralizing anti-TGF-β antibodies can block experimental bleomycin or radiation-induced pulmonary fibrosis.
Increased activity of the TGF-β pathway has also been implicated in fibrotic lung disease, glomerulosclerosis, and restenosis of cardiac vessels. Most TGF-β-mediated pathological changes appear to be attributed to the TGF-β1 isoform. The complexity of TGF-β1 function in humans is illuminated by hereditary disorders with generalized or cell-type specific enhancement or deficiency in either TGF-β1 itself or its signaling effectors. Mutations that increase the activity of the TGF-β pathway lead to defects in bone metabolism (i.e., Camurati-Engelmann disease) and in connective tissue (i.e., Marfan syndrome), and in aortic aneurysms (i.e., Loeys-Dietz syndrome), whereas mutations that lead to decreased activity of the TGF-β pathway correlate with cancer occurrence and prognosis. The role of TGF-β as a tumor suppressor in cancer is not straightforward, however, because TGF-β can also enhance tumor growth and metastasis, perhaps through its roles in immune suppression, cell invasion, epithelial-mesenchymal transition, or angiogenesis.
Despite the multiple essential functions of TGF-β, a single dose or short-term administration of a pan-TGF-β neutralizing antibody is reportedly well tolerated at doses that inhibit organ fibrosis or experimental carcinoma cell growth and metastasis, with no reported side effects in adult mice and rats. This treatment has shown therapeutic efficacy in inhibiting experimental fibrosis. Because of these promising results, single-dose phase I/II clinical trials using neutralizing pan-TGF-β antibodies have been performed or are ongoing for metastatic renal cell carcinoma, melanoma, focal segmental glomerulosclerosis, and idiopathic pulmonary fibrosis (Genzyme Corporation, genzymeclinicalresearch.com, last accessed Aug. 27, 2009). Careful targeting of the TGF-β pathway to minimize systemic effects is a highly desirable goal.